Engine RPM Drop Question

Today we crossed Lake Michigan for the first time with our 38 Commander - 90 miles. It took three winters to get her to this point which feels great. Two pictures are attached as she sits in Wisconsin now.

The vessel is powered by 454 crusaders. As we came across we heard some back fire (or could have been misfire) on the port engine. It was an intermittent single pop sound and we could see some mist coming out of the flame arrestor. This happened more at lower RPMs and less at 3,100. However the engine did periodically drop RPM at cruise speed, and the same thing started happening to the starboard engine, but back fire was not heard. Several times the RPM went to zero and the engine died. Both engines did it at different times during the trip. The engines have electronic ignition,electronic fuel pumps, new coils, and alternators. I am curious if anyone has suggestions on what could be the problem? I've got a new mechanic that's going to look at it. Also installed new fuel tanks over the winter.

Tim, that is worth checking. I also wonder if when they installed the new ignition they might have gotten one of the spark plug wires on the wrong cylinder. I also suspect a fault in carburetor. I am told there are some springs that can wear and the prior mechanic did some tweaking to them.

Chris my fuel tanks are brand new and the fuel is new so doubt fuel is the problem. New clean filters too.

My guess is that plug wire screw up is not the problem, it probably wouldn't stop engine and allow a start, btdt. Carbs that are screwed up are digital in that they work or don't and analog in between good to not so good. Coils are not so friendly, work, don't work so well or work just fine.

Ground or positive connection could be a heat problem as with the coils. If the ground connection is not tight and clean likewise the positive heat from loose connection could compromise the flow of smoke. See Lucas

Tim

Mark Regenhardt said:

Tim, that is worth checking. I also wonder if when they installed the new ignition they might have gotten one of the spark plug wires on the wrong cylinder. I also suspect a fault in carburetor. I am told there are some springs that can wear and the prior mechanic did some tweaking to them.

Chris my fuel tanks are brand new and the fuel is new so doubt fuel is the problem. New clean filters too.

It definitely sounds like an electrical issue, possibly multiple issues. Tim and Chris have the right idea.

The electric fuel pump circuit is where I would start my search. The popping through the intake sounds like fuel starvation causing the mixture to go lean.

I'm assuming that your tachs are now electric. If they are, the tachs going to zero before the engine is at a complete rest condition, indicates that the tachs are losing power just before total lose of ignition. This could be on the ground side and or the 12V+.

You also indicate that the engines act up more at lower rpm. This also sounds like it is telling us an electrical problem.

I suspect that there is an issue at a central ground point. The alternators are able to keep up with the load at the higher rpms as they are somewhat self excited at the higher rpm and the circuit resistance is low enough to keep everything sort of running. This is also tending to isolate the engine electrical system in front of the problem connection from the rest of the circuit. At the lower rpms, the resistance is high enough and the alternator output voltage low enough to become problematic.

Being that you installed new fuel tanks, I would be looking at where the tank ground and the electrical system ground comes together. It would also be a good idea to review what exactly was touched or modified during the tank replacement project. I'm guessing that both engines are suffering from a similar issue at a similar location.

The most likely connection issue will be a loose stud or screw. Second most likely issue will be a corroded connection either at a crimp on wire connection or possibly the battery terminals. Stressed wire connections are also a possibility.

This will be a challenging problem to locate. It is very likely that the problem connection will be running at a significantly elevated temperature which makes an infrared temperature gun a likely useful troubleshooting tool. Touch will also work but be careful of a very hot connection.

The trick will be to simulate the electrical loads that the circuits see while operating the boat and doing this at the dock with the engines off.

You might need to add an electrical load at the 12V+on the coil and the engine block as ground to cause enough current to flow to get the problem connection warm with the engines off but the ignition on. Something like a 12V bulb that will draw around 5-10A should do the trick.

I cannot emphasize enough the importance to be methodical and diligent in tracing the wiring circuits down. Best method is to print a copy of your boats schematic out and start marking off the circuits as you verify them for a high resistance connection.

If I remember correctly your cruising speed at 3100 It is probably bordering on the secondaries opening. Did you change the filter in the side body of the carb as well as the main fuel filter? If the filter is clogged you can create the fuel starvation that leans things out and can cause the popping and essentially the engine runs out of fuel and dies. It will restart immediately as there is enough fuel getting past the carb filter without the engine running at cruising speed or not at all to refill the bowl. I would not have believed it if it didn't happen to me and yes I checked ten different things before I was told to check those filters. Do you have fuel pump safety switches feeding your electric pumps that operate off of oil pressure? if so jump them out so you have a good electrical feed getting to your pumps and try again. Good luck. Keep us posted.

I would guess a fuel problem--starvation for some reason or another. Clogged fuel filter(s) would be most common. Safety shut off valves from the tank to pump could be another culprit if it has them. Older CCs including mine (1973) had an antisiphon safety pinhole(looks just like a very small carb jet) inserted in the fuel line right at the tanks to prevent fuel siphon into bilge. With low-tech setups --ie. mech pumps and carbs--the pinhole doesnt pull enough air into the system to matter. When I switched to elec pumps and EFI 8.1s--It mattered ALOT--and ran just as you described--until the pinhole was plugged. You mention elec fuel pumps and ignition--and carbs--so I do not know if this issue would present itself the exact same way--but I would check as the complete fix takes 5 min. If it is electrical--- grounds or feeds to fuel pumps sounds most likely to me.

+1 on anti siphon valve. After months of trouble shooting and part swapping/replacement I found it clogged. I’d think it more likely to manifest itself under load, but it’s so easy to check I’d be sure to add it to the list. - Bruno

Al Edwards said:

I would guess a fuel problem--starvation for some reason or another. Clogged fuel filter(s) would be most common. Safety shut off valves from the tank to pump could be another culprit if it has them. Older CCs including mine (1973) had an antisiphon safety pinhole(looks just like a very small carb jet) inserted in the fuel line right at the tanks to prevent fuel siphon into bilge. With low-tech setups --ie. mech pumps and carbs--the pinhole doesnt pull enough air into the system to matter. When I switched to elec pumps and EFI 8.1s--It mattered ALOT--and ran just as you described--until the pinhole was plugged. You mention elec fuel pumps and ignition--and carbs--so I do not know if this issue would present itself the exact same way--but I would check as the complete fix takes 5 min. If it is electrical--- grounds or feeds to fuel pumps sounds most likely to me.

Thanks everyone this is helpful advice. As mentioned above my tanks are brand new so I am doubtful of the anti-siphon cause. The problem was manifesting itself before I changed tanks. I suspect something eletrical related to the newer electric fuel pump wiring. As I recall the one time I ran the boat with mechanical pumps they ran fine. I am no expert so any simple to follow advice for how to diagnose a wiring problem of ground or otherwise would be greatly appreciated related to fuel pumps. One other doubt I have is could a damaged wire on the engine wiring harnesses cause this?

I have in the past checked the small engine fuel filter and cleaned it out, but the problem persisted. The Port Engine RPM gauge constantly fluttering 100 RPMs up or down with no perceived change in engine RPMs (going by sound) would seem to point to a voltage problem. My coils are new as well as the alternators fyi. I am on the boat this weekend and can try some things.

The 2018 Chris-Craft Commander Club National Rendezvous is in the books and it was a great time!!

The city of Mentor, Ohio and the Lawnfield Inn & Suites were wonderful hosts. A special Thanks to Rob Kneen, the local member sponsor, and Char Pike for the hours of hard work put in organizing and coordinating an amazing weekend.

The Rendezvous Photo Albums can be accessed through the links below, as well as, under Library, Photo Albums. We will continue to add links to other albums as they become available. Also below, you will find links to a couple articles written for the local newspaper.